Removal of Carbon Tetrachloride from Water by ... - ACS Publications

7 Removal of Carbon Tetrachloride from Water by Activated Carbon W A L T E R J. WEBER, JR. and MASSOUD PIRBAZARI

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Downloaded by UNIV OF BATH on July 3, 2016 | http://pubs.acs.org Publication Date: March 15, 1983 | doi: 10.1021/ba-1983-0202.ch007

University of Michigan, Department of Civil Engineering, Ann Arbor, MI 48109

Significant parameters associated with carbon tetrachloride adsorption by activated carbon were investigated to provide a basis for a more quantitative description of the process. Adsorption-desorption equilibrium studies suggested that the uptake of carbon tetrachloride by activated carbon is readily reversible. The Michigan Adsorption Design and Applications model (MADAM) was generally able to simulate and predict the performance of fixed-bed adsorbers for carbon tetrachloride removal from water. The effectiveness of activated carbon for carbon tetrachloride removal is likely to be adversely affected by the competitive adsorption and displacement effects of background organic substances. The configuration of the adsorber system is another factor that should be taken into consideration.

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ARBON TETRACHLORIDE ( C C l ) is a contaminant of many raw water 4

supplies and finished drinking waters (1-5). Because it has induced cancer in laboratory animals and is strongly suspected as a human carcinogen (6), its presence in potable waters is of concern (5). Activated carbon is capable of adsorbing C C l , but there is little known about the quantitative aspects of this adsorption and of factors that govern its effectiveness in water supply applications. The work reported here was designed to identify significant parameters associated with C C l adsorption by activated carbon and to provide a basis for more quantitative description of the process. 4

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Current address: University of Southern California, Los Angeles, CA 90007. 0065-2393/83/0202-0121$12.50/0 © 1983 American Chemical Society

McGuire and Suffet; Treatment of Water by Granular Activated Carbon Advances in Chemistry; American Chemical Society: Washington, DC, 1983.

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T R E A T M E N T O F W A T E R BY G R A N U L A R A C T I V A T E D C A R B O N

Experimental Solute. Baker Instra-Analyzed CC1 (Baker Chemical Co.; Phillipsburg, N.J.) was used. Experimental solutions were prepared in organic-free water (OFW) [deionized distilled water (DDW) passed through a Milli-Q water purifier (Millipore Corp., Bedford, MA)] or a solution of humic acids (Technical Grade, Aldrich Chemical, Milwaukee, WI) prepared in OFW. Adsorbent. Filtrasorb 400 granular activated carbon (Calgon Corp. Pittsburgh, PA) was sieved using a set of U.S. standard sieves, and selected size fractions were washed with D D W to remove leachable materials and carbon fines. The carbon was then dried to constant weight at 105°C for 12 h and immediately desiccated. After reaching room temperature, the carbon was stored in air-tight glass bottles. Analysis. Several important points were considered in the selection of the most suitable sampling, concentration, and analytical methods including: specificity, sensitivity, sample size, reproducibility, a detection limit requirement of < 1 /xg/L, and quickness and ease (7). A Varian 2700 equipped with a scandium tritide electron-capture detector was employed for gas chromatographic (GC) analysis of samples in which CC1 was present in very low concentration. The chromatograph was equipped with a 1.8-m X 0.3-cm (6-ft. X Vs-in.) stainless steel column packed with 20% SP 2100/ 0.1% Carbowax 1500 on 100-120 Supelcoport (Supelco, Inc.). To eliminate the problems associated with the characteristically small linear range of response of the scandium tritide electron-capture detector, more concentrated samples were analyzed using a Varian 1700 equipped with a similar column and a Tracor Instruments Model 700 Hall electrolytic conductivity detector, in the chlorine detection mode. Sample Collection and Concentration. DYNAMIC HEADSPACE. The stripping and preadsorption technique developed by Bellar and Lichtenberg (8) for volatile halogenated compounds was investigated as a potential technique for the analysis of CC1 at < 1 /xg/L. This technique, although highly efficient and reproducible, required special equipment for the stripping, adsorption, and desorption procedures and demanded considerable time for the large number of samples to be analyzed. STATIC HEADSPACE. The concentration of a gas dissolved in a liquid is, pursuant to Henrys law for dilute solutions, proportional at equilibrium to the partial pressure of that gas in the vapor phase, or headspace gas. Static headspace analysis was reported to be a sensitive technique for halomethane detection (9). As discussed by Weber et al. (10), the procedure is efficient and reproducible but highly temperature sensitive. Furthermore, the fact that solid, liquid, and vapor phases were all present in the adsorption experiments described here would have required a total mass balance estimation necessitating preparation of two sets of calibration curves. AQUEOUS INJECTION. Nicholson and Meresz (11) developed a technique whereby aqueous samples are directly injected into the chromatograph using a Sc H electron-capture detector. In the present work, however, aqueous injection was detrimental to the detector, substantially shortening its useful life. LIQUID-LIQUID EXTRACTION. Liquid-liquid extraction techniques have attracted interest since the development of specific modifications for the analysis of haloforms (e.g., 12, 13). In general, these techniques involve extraction into an organic solvent followed by G C measurement using an electron-capture detector; a detection limit of less than 1 /xg/L can be obtained.


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Removal of Carbon Tetrachloride from Water

The liquid-liquid extraction technique was the most attractive alternative for the present work, particularly with respect to compatibility with the adsorption experiments. In this procedure, 5-10-mL samples were mixed with 1-5 mL of Phillips 66 pure grade n-hexane and 0.5-1 g of Baker Analyzed sodium chloride in 10-20 mL vials fitted with Teflon-coated screw-on caps. The samples were shaken vigorously for 1 min, the hexane was allowed to separate, and solute analysis was performed by injecting a few microliters of the extract into a gas chromatograph. Adsorption Methodologies. EQUILIBRIUM STUDIES. Adsorption equilibrium experiments for CCI4 were using both bottle-point techniques and a miniadsorber-column procedure. Two distinct methods of preparing and analyzing the isotherms were evaluated in the bottle-point experiments; these were the static-headspace and liquidliquid extraction techniques already described. Each method yielded results that were reproduced easily by the other method. The static-headspace method consisted of placing different amounts of granular activated carbon in each of a series of 150-mL air-tight Hypo-Vials which had previously been cleaned carefully and heated in a muffle furnace at 400°C for 2 h. The vials were then filled with 100 mL of background solution spiked with CCI4, quickly sealed using Teflon-coated septa and crimped-on aluminum caps, and agitated at room temperature for 4 days to achieve equilibrium. Equilibrium vapor phase concentrations were determined by analysis of 0.10. 5 mL of the overhead gas from each vial and by comparison with a calibration curve prepared using known weights of compound dissolved in hexane. The vapor phase concentration of each system containing carbon was then compared with the vapor phase concentrations of appropriate carbon-free standards to yield, after correction for vapor solution equilibria, the corresponding residual aqueous phase concentration. The liquid-liquid extraction procedure was employed in adsorption experiments conducted in a manner similar to that already discussed, except that there were no vapor phase headspaces in the Hypo-Vials used to bring the activated carbon and CC1 solution to equilibrium. After equilibrium had been achieved, a 10-mL aliquot of each sample was subjected to a liquid-liquid extraction with the solvent system described earlier. Four sets of minicolumn studies were conducted to evaluate dynamic adsorption capacities of activated carbon wjth respect to four different influent concentrations of CC1 . These concentrations were chosen to cover a broad range, 1. e., 50, 200, 600, and 2,000 /ig/L. Feed solutions of CC1 were prepared in 12gallon (45-L) glass carboys using OFW. A second carboy of 5-gallon (19-L) capacity was used in each experiment to attentuate variations in influent concentration. A 0.5-mm (ID) capillary column was pierced through the Teflon-coated stopper of the larger carboy to prevent formation of a vacuum as solution was displaced. A precision volumetric pump (Flow Metering, Inc., Model #RP-SY) was employed to transfer the solution through Teflon tubing to a0.3-m (1-ft) long glass adsorber having an internal diameter of 1 cm and containing 1 g of 50/60 U.S. standard sieve size activated carbon. A flow rate of 1.6 gpm/ft (65 L/m /min) was maintained in all column, runs. The experimental apparatus and arrangement is illustrated in Figure L Effluent and influent samples were collected in glass vials and subjected to liquid-liquid extraction and G C analysis. RATE STUDIES. Completely mixed batch reactor (CMBR) adsorption rate experiments were conducted for CC1 —individually and in the presence of humic acids—in carefully sealed 2.6-L glass reactors of the type illustrated in Figure 2. Weighed and prewetted quantities of granular activated carbon were added to 4

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Removal ofCarbon Tetrachloride from Water

STIRRING ASSEMBLY (CHESAPEAKE)

LUER-LOK STOPCOCK (STAINLESS STEEL)

31/15

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J JOINT

GLASS DISPLACEMENT PLUNGER

24/40 J JOINT


TEFLON BUSHING

2H/HQ $ JOINT (2)

TEFLON COATED SEPTUM

10" GAGE SYRINGE

2000 ML RESIN REACTION KETTLE (CORNING) Figure 2. Scheme of the CMBR appropriate experimental solutions (pH 7; O F W or 5 mg/L humic acid prepared in O F W background solution) in the vapor phase free reactors. The carbon was dispersed by a motor-driven glass stirrer and 5-mL samples withdrawn at fixed time intervals using a 10-mL hypodermic syringe. A displacement plunger built into the reactor eliminated introduction of headspace by sample volume displacement Both dynamic headspace and liquid-liquid extraction techniques were compatible with the methods employed in the CMBR adsorption rate studies with CC1 . The liquid-liquid extraction procedure was used for the majority of the studies because it involved less time. The reactor systems and experimental procedures devised for these rate studies were particularly successful. The reactor design eliminated potential solute 4


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loss by volatilization and evaporation. Furthermore, because the solution in the CMBR was in contact with only with glass and Teflon, no measurable sorbate loss to containing surfaces was encountered. Elimination of these losses is important when rate studies are performed with very dilute solutions of volatile compounds. COLUMN STUDIES. Column studies were conducted for evaluation of breakthrough profiles for CC1 . Saturated solutions of CC1 were prepared in 200-L airtight stainless steel feed tanks. After reaching saturation, the experimental aqueous feed solutions were kept in constant contact with CC1 vapor. Saturated solution was withdrawn by high precision volumetric pumps, diluted to appropriate concentration with high purity water, and then passed through glass mixing chambers to the adsorbers. All transfer tubing was stainless steel and/or Teflon. Each adsorber was comprised of a 1.8-m (6-ft) long glass column with an internal diameter of 3 cm containing 250 g of 16/20 U.S. standard sieve size activated carbon. Influent and effluent samples were collected in 20-mL vials. Care was taken during sample transfer to minimize CC1 loss by volatilization. The samples were then concentrated using the liquid-liquid extraction technique and analyzed by G C . Figure 3 illustrates the experimental apparatus and arrangement. Two major problems were encountered during the experiments. First, the influent concentration fluctuated considerably, perhaps due to blockage of the carbon tetrachloride solution transfer tubing. This problem necessitated frequent maintenance and readjustment. Second, biological growth on carbon particles in the adsorber generated considerable headloss, requiring periodic air-scouring and backwashing. Various inhibitors for preventing biological growth were evaluated to permit singular determination of appropriate adsorption parameters, with only marginal success. 4

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Results and Discussion Adsorption Equilibria. tested, and the

Several different isotherm models were

Freundlich equation was found to provide the most

straightforward and generally satisfactory method for fitting and

de-

scribing the experimental data. Equilibrium data derived from both bottle-point and minicolumn experiments are presented in Figure 4; some small differences in adsorption capacities obtained by the two methods may be observed. Figure 5 illustrates that pH has no significant effect on the equilibrium adsorption characteristics of CC1 . No dependency of equilibrium capacity on carbon 4

particle size was experienced (Figure 6). Figure 7 demonstrates that initial concentration also has no significant effect on the position of the isotherm. Figure 8 compares isotherm data for CC1

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in O F W background

solution and in the presence of 5 mg/L humic acid in OFW. Comparative isotherm data for O F W background solutions and Ann Arbor tap water are presented in Figure 9. While the equilibrium capacities for adsorption of CC1 appear from the data presented in these figures to be little affected 4

by background constituents characteristic of raw water sources and finished drinking water supplies, these data pertain only to equilibrium conditions and do not address potential competitive effects relating to adsorption rates. Moreover, they pertain only to systems in which initial


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WEBER AND PIRBAZARI Removal of Carbon Tetrachloride from Water127

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Removal of Carbon Tetrachloride from Water by ... - ACS Publications

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